1. Field of the Invention
The present invention relates to a laser light generating apparatus for generating laser light having a wavelength converted by a non-linear optical crystal by introducing fundamental laser light from the outside to an optical resonator or generating fundamental laser light within the optical resonator.
2. Description of the Related Art
Since the power density of laser light inside an optical resonator is high, effective conversion in wavelength is expected. Among such known optical resonators for a laser light generating apparatus are an external resonance type second harmonic generator (SHG) and an internal resonance type SHG.
The internal resonance type SHG has such a structure that a laser medium and a non-linear optical crystal are arranged in one optical resonator, and effective conversion in wavelength can be realized by satisfying phase matching conditions between fundamental laser light generated by the laser medium and second harmonic laser light converted by the non-linear optical crystal.
The external resonance type SHG has such a structure that a non-linear optical crystal is arranged in a second optical resonator which is separated from a laser resonator for generating fundamental laser light, and the conversion in wavelength to the second harmonic is carried out by resonance of the fundamental laser light in the second optical resonator.
In such external resonance type SHG, the power density in the optical resonator can be is increased by some hundreds times the power density of incident light by setting the fines value (Q value) representing the sharpness of resonance of the optical resonator to a large value such as 10 to 1000, hence increasing the efficiency of conversion in wavelength by the non-linear optical crystal in the optical resonator.
FIGS. 8A and 8B are configuration views of external resonance type SHGs. More particularly, FIG. 8A shows an example in which a Z-ring optical resonator is used and FIG. 8B is an example in which a triangle-ring optical resonator is used. Referring to FIG. 8A, the optical resonator has four reflecting mirrors M1 to M4 arranged in a so-called bow-tie pattern where the optical axes of the resonator cross each other, and a non-linear optical crystal 50 is displaced between the two reflecting mirrors M1 and M2. Fundamental laser light F emitted from an external laser apparatus is introduced through the reflecting mirror M4 to the interior of the optical resonator to resonate. After passing the non-linear optical crystal 50, the fundamental laser light is converted to a second harmonic, reflected by the two reflecting mirrors M2 and M3, and released out from the reflecting mirror M3 as second harmonic laser light S.
Referring to FIG. 8B, the optical resonator has such a structure that three reflecting mirrors M1 to M3 are arranged so that the optical axes of the optical resonator forms a triangle, and a non-linear optical crystal 50 is displaced between the two reflecting mirrors M1 and M2. Fundamental laser light F emitted from an external laser apparatus is introduced through the reflecting mirror M1 to the interior of the optical resonator to resonate. After passing the non-linear optical crystal 50, the fundamental laser light F is converted to the second harmonic, and released outside from the reflecting mirror M2 as the second harmonic laser light S.
In this manner for increasing the fines value of the optical resonator, it is preferable to minimize the number of the reflecting mirrors to restrain loss of the laser light in the reflecting mirrors as small as possible. The reflecting mirror M3 shown in FIG. 8B tends to decrease the reflection coefficient of incident light due to a large incident angle of the incident light, with the result that the fines value of the optical resonator is decreased. In particular, in the case of generating a harmonic of the ultraviolet spectrum, since the non-linear constant of the non-linear optical crystal 50 is decreased, thus decreasing the efficiency of wavelength conversion. Hence, the ring-type resonator with higher fines value composed of four reflecting mirrors as shown in FIG. 8A is often used as the optical resonator for generating ultraviolet rays.
On the other hand, the efficiency of the wavelength conversion in the non-linear optical crystal is increased proportional to the power density of the fundamental laser light F. Accordingly the non-linear optical crystal 50 is thus displaced at a beam waist where the power density is maximum.
Additionally phase matching conditions in the non-linear optical crystal are important as well as a large acceptance angle depending on the phase matching conditions. The acceptance angle reaches a maximum when non-critical phase matching is established, i.e. the crystal axis of the non-linear optical crystal is consistent with the optical path of the laser light. It is ideal but is hardly feasible to locate the non-linear optical crystal so that such non-critical phase matching is achieved through assembling and adjusting pertinent components at higher accuracy. Commonly, a compromise of critical phase matching is employed in which the phase matching is established within a given angle from the crystal axis. Such critical phase matching is however unfavorable in the respect of allowance. Also, the allowance of phase matching will vary between two orthogonal directions of the optical path.
For example, there is not known such a non-linear optical crystal as to achieve non-critical phase matching in producing a second harmonic of the ultraviolet spectrum from its fundamental wave of a substantially 500 nm wavelength by wavelength conversion. Generally, beta-barium borate (BBO) crystal is used as the non-linear optical crystal for angular phase matching which is one of critical phase matching modes.
The angular phase matching is explained in more detail. When the fundamental wave is incident on a BBO crystal along the horizontal direction, phase matching is achieved with the c-axis of the BBO crystal extending on the horizontal plane. Two of the acceptance angles are provided on the vertical plane and on the horizontal plane where the optical path of the fundamental wave lies. Assuming that a second harmonic is produced from the fundamental wave of a 532 nm wavelength in type I of phase matching, the allowance (a product of the acceptance angle and the length of the crystal) in the .phi. direction from the c-axis of the BBO crystal is 0.6 (deg.multidot.cm) which is much greater than the allowance 0.016 (deg.multidot.cm) in the .theta. direction vertical to the .phi. direction. This causes the efficiency of wavelength conversion to be hardly increased even if the laser light is converged through a greater angle than the acceptance angle in the .theta. direction. Hence, the laser light may be moderately converged in the .theta. direction so that its converging angle is smaller than the acceptance angle.
FIGS. 9A through 9D are views showing convergence patterns of the laser light. More specifically, FIGS. 9A and 9B are cross sectional views of .theta. plane and .phi. plane, respectively in one convergence pattern and FIGS. 9C and 9D are cross sectional views of .theta. plane and .phi. plane, respectively, in another convergence pattern. As shown in FIG. 9, a solid line P represents a profile of the location where the intensity of the laser light beam is decayed from its maximum to e .sup.-2 (e being a base of natural logarithm). Also, the broken line Q is an asymptonic line of the beam profile and the solid line R represents a range of the acceptance angle.
In case of Gaussian beam with the distribution of the light intensity representing normal distribution, there is a certain relationship between the converging angle and the diameter of a beam waist. Assuming that the converging angle or an open half angle between the center line and the asymptotic line is .alpha., the diameter of the beam waist is .omega., the wavelength of the laser light is .lambda., and the refractive index is n, the following equation is established. EQU .alpha.=.lambda./(.pi..multidot..omega..multidot.n) (1)
It is common that an acceptance angle .delta..theta. in .theta. direction and an acceptance angle .delta..theta. in .phi. direction are different from each other on a c-axis of the non-linear optical crystal. In the case where the acceptance angle .delta..phi. is, for example, greater than the acceptance angle .delta..theta. (.delta..phi.&gt;&gt;.delta..theta.) as shown in FIGS. 9A and 9B, when axially symmetrical laser light is used, the converging angle .alpha.2 is substantially consistent with the acceptance angle .delta..phi. on the .phi. plane of FIG. 9B, so that effective wavelength conversion is achieved. However, since the converging angle .alpha.1 is much greater than the acceptance angle .delta..theta. on the .theta. plane of FIG. 9A, only a part of the laser light within the acceptance angle .delta..theta. contributes to the wavelength conversion. Accordingly, the far-field pattern of a resultant second harmonic released from the non-linear optical crystal is a narrow oval shape extending along the .theta. direction with the result of low efficiency of the wavelength conversion.
On the other hand, as shown in FIGS. 9C and 9D, when the beam waist diameter .omega.3 is set large by moderately converging the laser light so that the converging angle .alpha.3 in the .theta. direction is small, the converging angle .alpha.3 is substantially consistent with the acceptance angle .delta..theta.. As the result, the efficiency of the wavelength conversion will be maintained high in both the planes.
The laser light having such a profile as shown in FIGS. 9C and 9D is easily realized by converging intensively in one direction and moderately in another direction vertical to the direction. This allows the use of an optical converging system which comprises a combination of a spherical concave lens and a cylindrical lens ("Applied Physics", vol. 61, No. 9, p. 931, 1992)
FIG. 10 is a configuration view of an example of a conventional laser light generating apparatus. This is disclosed in the above-mentioned reference, in which an optical resonator of argon ion laser is composed of a plasma tube 51 filled with argon ions and two reflecting mirrors 52 and 53. The optical resonator contains therein a non-linear optical crystal or BBO crystal 54 for wavelength conversion to produce second harmonic laser light S which is released outside from the reflecting mirror 53.
Furthermore, there are provided a spherical lens 57 and two cylindrical lenses 55 and 56 for controlling the convergence status of the laser light in the BBO crystal 54.
The spherical lens 57 has convergence power on two planes vertical to and parallel to the plane of a sheet of FIG. 10 and the cylindrical lenses 55 and 56 have convergence power only on the plane parallel to the sheet. Accordingly such a convergence status that the beam waist diameter and the converging angle of the laser light are different between the two planes orthogonal to each other is realized in the BBO crystal 54.
However, as shown in FIG. 10, the arrangement of optical parts such as a converging lens in the optical resonator invites an increase in the loss of the laser light, and as a result the fines value of the resonator remarkably decreases, resulting in reduction in wavelength conversion efficiency. On the other hand, if the acceptance angle of the non-linear optical crystal is different between the two planes orthogonal to each other, it is difficult to suitably maintain the convergence status of the laser light with no use of extra optical devices.